You can probably sell the one you got for 150ish bucks and get a 6970 or a gtx 570.
Of course you can always pay the 150 and get an extra 5870 if you can find one.
Getting 2x6950 1gb would be awesome but i doubt anybody really needs this much of gpu power. TBH my single 6950 plays everything on ultra and looking at 3d mark results shows that your processor is slightly behind in score (look at average histogram charts)http://3dmark.com/3dm11/940903http://3dmark.com/3dm11/1903564
These are for 2x5870: http://3dmark.com/3dm11/1225989
though keep in mind that you will be getting micro stuttering
This is for a single 6970: http://3dmark.com/3dm11/2016701
There are not many entries but the numbers are so sexy. Choice is yours.